Radio Control: Scale
Dennis Crooks P.O. Box 98 Big Rock, IL 60511
Introduction
Over the years just about every conceivable aspect of RC scale flying has been covered in the magazines: basics of building, covering, and flying; balsa-growing jokes; and fiberglass weaving techniques. One area that has seen growing acceptance in sport and competition scale flying is the use of ducted fans. That introduces a new set of concerns and techniques. The notes below address the most common issues.
Model selection
Model selection for ducted-fan scale aircraft is no more restricted than for propeller-driven models, but choose wisely according to your experience level.
- Avoid jets that are inherently challenging if you're inexperienced — your second trainer should not be the inevitable P-51 or a high-performance jet.
- If the full-scale aircraft was designed as a trainer, the model will often be a good trainer too.
- Many fliers prefer Navy jets. Navy aircraft are designed to operate from short fields (carriers) while carrying fuel and ordnance; that tends to produce desirable traits in a scale model:
- Strong landing gear
- A wing capable of carrying loads across the speed envelope
Emotion and personal preference will often decide your choice, but consider these practical points.
Inlet design and performance
With more powerful fans now available, inlet size is less restrictive than it once was — but you can't simply cut a hole in the fuselage and expect optimal performance. Small inlets can work well if they are carefully designed.
- For scale accuracy, I prefer to accept a small performance loss rather than compromise appearance.
- Small inlets require special attention:
- The inlet lip should be as rounded as possible.
- Internal air routing must be subtle and smooth.
- Example: my Yellow Aircraft F-16 uses an exact-scale inlet (made from General Dynamics drawings). Takeoff performance is slightly reduced compared with an exaggerated inlet, but it still gets airborne in under 150 ft.
- Some airframes, such as the Yellow Aircraft F-14 Tomcat at 1/6 scale, are nearly ideal in size. Sig Kadet ducted fans and the Byron Originals MiG-15R can operate from 60+-size engine fields.
Weight saving and construction techniques
Weight saving is important, but fashion-driven lightening can be counterproductive.
- Avoid excessive or careless cutting:
- Poking holes in foam wings or over-lightening sheeted wings can ruin appearance and weaken landing-gear mounts.
- Use conscientious woodworking and glue practices rather than gimmicks.
- Recommended practices:
- Drill lightening holes only in non-stress areas.
- Provide bulkhead access where bulkheads create hard-to-reach joints.
- Tack-glue fuselage shells, stand the model on end, and run a small bead of polyester epoxy (for example, Epoxid-Klebeharz) around the joint. Wipe away excess — glue smeared away from the joint is wasted dead weight.
Sealing and fuel-proofing
Because fuel and exhaust are contained inside ducted-fan fuselages, sealing and protection of internal wood parts is crucial.
- After glue-up, soak wood parts in thin Zap (thin CA) to allow adhesive to penetrate and seal the wood.
- Paint the inside of the fuselage to seal wood, improve appearance, and make failed bonds easy to spot.
- These steps are easy to overlook but will help ensure the model survives high-speed passes and strenuous maneuvers.
Control surfaces and rigidity
Scale appearance often discourages visible control horns and pushrods, but with jets this can be unavoidable for safety and rigidity.
- Control surface hinges must be snug and as gapless as possible to prevent flutter and for realism.
- Ailerons and flaps are often connected to wing-mounted servos via exposed horns and pushrods for maximum rigidity.
- Fast flights increase the likelihood of flutter; ensure hinges and linkages are robust.
Flying tips and competition etiquette
Ducted-fan models have some operational advantages and require considerate behavior at contests.
- Advantages:
- Most jets have nose wheels, which help with crosswind control.
- There is no propwash to keep controls effective at very low speeds.
- Courtesies and safety:
- Do not run or rest your engine in the pits while competition flights are underway — pilots and judges need to hear calls and instructions.
- When starting, be aware of exhaust direction. Jet exhaust can have high velocity and blow hats, gear, or nearby models.
- Start and run-up models a respectful distance from the judges. Jet noise can be intense (for example, an F-14 with two high-RPM engines is very loud), and judges rarely wear hearing protection.
- Treat judges and fellow competitors with respect — it can affect your contest experience and score.
Conclusion
Ducted fans open new possibilities for scale modelers without fundamentally changing model selection principles. Accepting small performance trade-offs for scale accuracy, paying attention to inlet design, following sound construction and sealing practices, and using rigid control linkages will make ducted-fan scale flying both rewarding and safe. Respectful behavior at contests and thoughtful preflight practices complete the picture.
Transcribed from original scans by AI. Minor OCR errors may remain.
